using python to encapsulate part of a string after 3 commas

I am trying to create a python script that adds quotations around part of a string, after 3 commas

So if the input data looks like this:

1234,1,1/1/2010,This is a test. One, two, three.

I want python to convert the string to:

1234,1,1/1/2010,"This is a test. One, two, three."

The quotes will always need to be added after 3 commas

I am using Python 3.1.2 and have the following so far:

i_file=open("input.csv","r")
o_file=open("output.csv","w")

for line in i_file:
        tokens=line.split(",")
        count=0
        new_line=""
        for element in tokens:
                if count = "3":
                        new_line = new_line + '"' + element + '"'
                        break
                else:
                        new_line = new_line + element + ","
                        count=count+1

        o_file.write(new_line + "\n")
        print(line, " -> ", 开发者_JAVA技巧new_line)

i_file.close()
o_file.close()

The script closes immediately when I try to run it and produces no output

Can you see what's wrong?

Thanks

---

Having addressed the two issues mentioned in my comment above I've just tested that the code below (edit: ALMOST works; see very short code sample below for a fully tested and working version) for your test input.

i_file=open("input.csv","r")
o_file=open("output.csv","w")

for line in i_file:
    tokens=line.split(",")
    count=0
    new_line=""
    for element in tokens:
        if count == 3:
            new_line = new_line + '"' + element + '"'
            break
        else:
            new_line = new_line + element + ","
            count=count+1

    o_file.write(new_line + "\n")
    print(line, " -> ", new_line)

i_file.close()
o_file.close()

Side note:
A relatively new feature in Python is the with statement. Below is an example of how you might take advantage of that more-robust method of coding (note that you don't need to add the close() calls at the end of processing):

with open("input.csv","r") as i_file, open("output.csv","w") as o_file:
    for line in i_file:
        tokens = line.split(",", 3)
        if len(tokens) > 3:
            o_file.write(','.join(tokens[0:3]))
            o_file.write('"{0}"'.format(tokens[-1].rstrip('\n')))

---

Shorter but untested:

i_file=open("input.csv","r")
o_file=open("output.csv","w")

comma = ','
for line in i_file:
    tokens=line.split(",")
    new_line = comma.join(tokens[:3]+['"'+comma.join(tokens[3:])+'"'])
    o_file.write(new_line+'\n')
    print(line, " -> ", new_line)

i_file.close()
o_file.close()

---

Perhaps you should consider using a regular expression to do this? Something like

import re
t = "1234,1,1/1/2010,This is a test. One, two, three."
first,rest = re.search(r'([^,]+,[^,]+,[^,]+,)(.*)',t).groups()
op = '%s"%s"'%(first,rest)
print op

1234,1,1/1/2010,"This is a test. One, two, three."

Does this satisfy your requirements?

---

>>> import re
>>> s
'1234,1,1/1/2010,This is a test. One, two, three.'
>>> re.sub("(.[^,]*,.[^,]*,.[^,]*,)(.*)" , '\\1\"\\2"' , s)
'1234,1,1/1/2010,"This is a test. One, two, three."'

import re
o=open("output.csv","w")
for line in open("input.csv"):
    line=re.sub("(.[^,]*,.[^,]*,.[^,]*,)(.*)" , '\\1\"\\2"' , line)
    o.write(line)
o.close()